GaN-based LEDs with high external quantum efficiency
演讲课题：GaN-based LEDs with high external quantum efficiency
We report MOCVD growth of cascaded GaN-based optoelectronic devices with tunnel junction layers (TJLs) and the characterizations of these devices.In order to overcome the efficiency droop problem and improve externl quantum efficiency of light emitting diodes (LEDs), we first fabricated GaN-based multiquantum well LED with tunnel junction (TJ)-cascaded active region. Compared with the conventional LED, it was found that we could achieve 35% higher light output power from the TJ LED due to the repeated use of electron and holes for photon generation. It was also found that external quantum efficiency (EQE) drooped by 36.9% and 35.0% for the TJ LED and the conventional LED, respectively, as we increased the injection current density to 80 A/cm2. Furthermore, it was found that forward voltages measured with an injection current density of 20 A/cm2 were 8.94 V for the TJ LED. The large forward voltage observed from the TJ LED should be attributed to the large TJ resistance . Secondly, we tried to reduce the TJ resistance by fabricating the cascaded GaN LEDs with hybrid TJL. Compared with the conventional LED, it was found that we could enhance the light output power by 80% from the LED with hybrid TJL. It was also found that the TJ resistances were only 1.95×10-3 Ω·cm2 for the LED with hybrid TJL. It was also found that the use of hybrid TJL could result in smaller efficiency droop. These improvements could be attributed to the larger polarization charges induced at the AlGaN/InGaN interface which could enhance the tunneling current. We also studied the use of TJ structure to cascade a green multiquantum well (MQW) active region and a blue MQW active region, and the fabrication of GaN-based dual-color LEDs. It was found that light output power observed from the TJ cascaded LED was only slightly smaller than the summation of those observed from the green LED and the blue LED. This suggests that most of the injected carriers could tunnel through the TJ and could be repeatedly used for photon generation . We also coated red phosphors on the green/blue dual-color TJ LEDs to fabricate the white LEDs. It was found that the CIE color coordinates of “dual-color TJ LED + red phosphor” locates at (0.30, 0.35) with the color temperature, TC=4306 K and color-rendering index, CRI =68. This suggests that such device is potentially useful as the white LED . References  S. J. Chang, W. H. Lin and C. T. Yu, "GaN-based multiquantum well light-emitting diodes with tunnel-junction-cascaded active regions", IEEE Electron. Dev. Lett., Vol. 36, No. 4, pp. 366-368, April 2015. S. J. Chang, W. H. Lin and W. S. Chen, "Cascaded GaN light-emitting diodes with hybrid tunnel junction layers", IEEE J. Quan. Electron., Vol. 51, No. 8, Art. no. 3300505, August 2015. W. H. Lin, S. J. Chang and W. S. Chen, "GaN-based dual-color light-emitting diodes with a hybrid tunnel junction structure", IEEE/OSA J. Display Technol., Vol. 12, No. 2, pp. 165-170, February 2016.  K. T. Lam, W. H. Lin, S. C. Shei, N. M. Lin, W. S. Chen and S. J. Chang, "White-light emission from GaN-based TJ LEDs coated with red phosphor", IEEE Electron. Dev. Lett., Vol. 37, No. 9, pp. 1150-1153, September 2016.